The use of aluminum sheet material is increasing steadily in the manufacture of light-weight automobiles and similar vehicles. For skin applications, such as hoods, trunk lids and fenders, alloy AA6111 is becoming the preferred choice of the North American automakers. This alloy, developed by Alcan, the assignee of the present application, has good forming properties prior to a paint/bake cycle and good dent resistance after forming and painting. For body structure construction, however, the alloy is too strong and the medium strength AA5754 alloy has been recommended for this application (so-called 5000 series aluminum alloys have magnesium as the major alloying element and are generally softer than the 6000 series aluminum alloys). For the most part, 5000 series alloys are well suited for manufacturing all-aluminum body structures, but somewhat higher strength would be advantageous and there is a concern about the recycling of vehicles containing both 5000 and 6000 series alloys since they are chemically incompatible.
Aluminum alloys suggested for use in the automotive industry include those disclosed in the following U.S. Pat. Nos.: 4,082,578 to Evancho et al.; 4,589,932 to Park; 4,784,921 to Hyland et al.; and 4,840,852 also to Hyland et al.
Unfortunately, no known aluminum alloys that are chemically compatible with skin alloy AA6111 satisfy the demands of structural applications in vehicles, including adequate (but not too high) strength and an ability to collapse uniformly upon impact.